An escapement broadly answering the description that has just been given was disclosed in the work entitled “La Montre: principes et méthodes de fabrication”, by George Daniels, Scriptar Editions S.A., La Conversion, Lausanne 1993. This escapement is explained at pages 236 to 239 of said work and a drawing is reproduced in FIG. 1 of this document to illustrate this prior art.
As FIG. 1 shows, the Daniels escapement includes a brake lever 40 located between two escape wheels A and B. This brake lever 40 is articulated on a pivot 41 and is controlled by an impulse pin 42 of roller 43. Roller 43 is secured to a balance that is not shown and the pivot 44 of the roller is substantially equidistant from the axes of rotation of wheels A and B. Roller 43 carries two pallet stones 45 and 46 arranged for receiving impulses respectively from wheel A then wheel B. In the configuration shown in FIG. 1, wheels A and B are locked. Brake lever 40 carries three locking pallet stones C, S1 and S2. Wheel A is locked by locking pallet stone C and wheel B by locking pallet stones S1. This is a first stable state of equilibrium in which brake lever 40 is inclined towards wheel A. Roller 43 is rotating in the direction of arrow 47. When the impulse pin 42 thereof penetrates fork 52, brake lever 40 rotates in the anticlockwise direction and passes through a median position, which releases tooth 51 from wheel A. The latter then rotates in the anticlockwise direction and imparts an impulse, via its tooth 48, to pallet stone 45 of roller 43. As roller 43 continues its travel in the direction of arrow 47, the brake lever then inclines towards wheel B, which releases tooth 50 from locking pallet stone S1, then almost simultaneously engages tooth 49 on locking pallet stone C of brake lever 40. There is a lock transfer from S1 to C, wheel B then rotating over a small angle in the clockwise direction during the transfer. In its median position, locking pallet stone S2 of brake lever 40 intersects the trajectory of the teeth of wheel A and in particular tooth 53, which then finally comes to rest on S2 when impulse pin 42, rotating in the direction of arrow 47 has exited fork 52. From now on, there is a second stable state of equilibrium in which brake lever 40 is inclined towards wheel B. This escapement system will thus be called bistable.
As ingenious as it may appear, the escapement that has just been described suffers from non-negligible drawbacks By the admission of the author of the aforecited work, this escapement is difficult to construct and the pivot holes, locking pallet stones, impulse pin and impulse pallet stones have to be precisely implanted to prevent any loss of efficiency in operation. Moreover, the escapement requires three locking pallet stones, whereas the escapement of the present invention needs only two locking pallet stones, as will be seen below. In this prior art, it was seen that there is a lock transfer from one pallet stone S1 to the other C (and in the other direction from S2 to C) allowing the wheel B concerned also to escape (and in the other direction wheel A) for a short moment, which does not occur without disturbing the operation of the system. Finally, the bistable system of the prior art appears quite different to implement from the monostable system proposed by the present invention, as the bistable system does not lead to an actual detent escapement.